A man, a woman and a boy can do a piece of work in 6, 9 and 18 days, respectively. How many boys should assist one man and one woman to do the work in one day?

13 boys are needed to finish the work in one day.

Rates per day: man 1/6, woman 1/9, boy 1/18.

One man + one woman + x boys must total 1: 1/6 + 1/9 + x/18 = 1.

Common denominator 18: 3/18 + 2/18 + x/18 = 1.

5 + x = 18, so x = 13.